We had our appointment today.
Radiographs showed that he was not bloated, like he feels. In fact, there was very little gas in his GI tract. There was a slightly radio opaque bit that looked like it could be the partially digested pinky skeleton... so perhaps he's not digesting well.
They gave him an enema, which successfully produced a stool. They'll send that to the lab to look for parasites. He also feels less balloony now that he's pooped...
Beyond that, they can't tell me why he seems weak in his back half. He can feel, but he doesn't have the same muscle as he does in the front.
He remains bright and alert.